David Eric Casebolt resigned from McKinney Police Department after almost 10 years on the force. His resignation was confirmed by his lawyer, Jane Bishkin.

A viral video showed him pushing a bikini-clad girl to the ground and brandishing his gun at other black teenagers after he and other officers responded to complaints about the pool party at a community-owned swimming pool in the Dallas suburb of McKinney.

McKinney police chief Greg Conley had placed the former Texas state trooper on administrative leave after the incident and said his actions were "indefensible".

Ms Bishkin declined to say where 41-year-old was and said he had received death threats.

Yesterday thousands of demonstrators rallied outside a local primary school in protest over the white officer's actions.

Some held signs that included the phrases "My skin colour is not a crime" and "Don't tread on our kids".

In the video, as police broke up the crowd, Casebolt pulled the 15-year-old girl to the ground, then used his knees to pin her down. He also pointed his gun at other teenagers and swore at them.

The girl claimed the officer told her to walk away but forced her down after "he thought we were saying rude stuff to him", according to an interview she gave to television station KDFW.

"He grabbed me, twisted my arm on my back and shoved me in the grass and started pulling the back of my braids," Dajerria Becton told the station. "I was telling him to get off me because my back was hurting bad."

The National Association for the Advancement of Coloured People has asked the Department of Justice to review the procedures of McKinney police department.

Gary Bledsoe, president of the Texas chapter of the NAACP, said the organisation has so far stopped short of asking for a federal investigation.

He said a review of department policies is needed to ensure officers are responding appropriately to calls involving minorities.

Mr Bledsoe also said independent investigators must review the response to the pool party.

Mr Conley said Casebolt had not been pressured into quitting.

He said: "Our policies, our training and our practices do not support his actions."

In all, 12 officers responded to the report of fights and a disturbance at the pool party at the Craig Ranch North Community Pool in an affluent area of western McKinney.

"Eleven of them performed according to their training," Mr Conley said. Casebolt did not, he added.

"He came into the call out of control and the video showed he was out of control during the incident," Mr Conley said.

Despite the resignation, Casebolt's actions remain under investigation and no decision has been made on whether charges will be filed against him, Mr Conley said.

Charges of interfering with an officer and evading arrest against the only man arrested during the incident have been dropped. Everyone else was released.

The resignation is a step in the right direction, said Dominique Alexander, president of the Dallas area Next Generation Action Network and organiser of the demonstrations.

"We still need a serious investigation into the charges that need to be brought against him in this matter," Ms Alexander said, adding that Casebolt should be drug tested.